package kcp

import (
	"hash/fnv"

	"v2ray.com/core/common/alloc"
	"v2ray.com/core/common/serial"
	"v2ray.com/core/transport/internet"
)

type SimpleAuthenticator struct{}

func NewSimpleAuthenticator() internet.Authenticator {
	return &SimpleAuthenticator{}
}

func (v *SimpleAuthenticator) Overhead() int {
	return 6
}

func (v *SimpleAuthenticator) Seal(buffer *alloc.Buffer) {
	buffer.PrependFunc(2, serial.WriteUint16(uint16(buffer.Len())))
	fnvHash := fnv.New32a()
	fnvHash.Write(buffer.Bytes())
	buffer.PrependFunc(4, serial.WriteHash(fnvHash))

	len := buffer.Len()
	xtra := 4 - len%4
	if xtra != 0 {
		buffer.Slice(0, len+xtra)
	}
	xorfwd(buffer.Bytes())
	if xtra != 0 {
		buffer.Slice(0, len)
	}
}

func (v *SimpleAuthenticator) Open(buffer *alloc.Buffer) bool {
	len := buffer.Len()
	xtra := 4 - len%4
	if xtra != 0 {
		buffer.Slice(0, len+xtra)
	}
	xorbkd(buffer.Bytes())
	if xtra != 0 {
		buffer.Slice(0, len)
	}

	fnvHash := fnv.New32a()
	fnvHash.Write(buffer.BytesFrom(4))
	if serial.BytesToUint32(buffer.BytesTo(4)) != fnvHash.Sum32() {
		return false
	}

	length := serial.BytesToUint16(buffer.BytesRange(4, 6))
	if buffer.Len()-6 != int(length) {
		return false
	}

	buffer.SliceFrom(6)

	return true
}